The invention relates to a process for the preparation of mouldings, especially optical lenses and in particular contact lenses, to a corresponding device for the preparation of mouldings, and to the mouldings that have been prepared or are obtainable by the process or using the device, especially optical lenses and in particular contact lenses, in accordance with the preamble of each independent patent claim.
Contact lenses, which are to be manufactured economically in large unit numbers, are preferably manufactured by the so-called mould or full-mould process. In these processes, the lenses are manufactured into their final shape between two moulds, so that there is no need to subsequently finish the surfaces of the lenses, nor to finish the edges. Such moulds consist of a female mould half and a male mould half, the cavity being formed between the two mould halves defining the shape of the moulding. Mould processes are described for example in PCT application no. WO/87/04390 or in European patent application EP-A-0 367 513.
In these known mould processes, the geometry of the contact lenses to be manufactured is defined by the mould cavity. The edge of the contact lens is likewise formed by the mould which normally consists of two mould halves. The geometry of the edge is defined by the contour of the two mould halves in the area in which they touch one another.
To prepare a contact lens, first of all a certain amount of a flowable starting material is placed in the female mould half. Afterwards, the mould is closed by placing the male mould half thereon. Normally, a surplus of starting material is used, so that, when the mould is closed, the excess amount is expelled into an overflow area outwardly adjacent to the mould cavity. The subsequent polymerisation or crosslinking of the starting material takes place by radiation with UV light, or by heat action, or by another non-thermal method. Both the starting material in the mould cavity and the excess material in the overflow area are thereby hardened. In order to obtain error-free separation of the contact lens from the excess material, a good seal or expulsion of the excess material must be achieved in the contact zone of the two mould halves. Only in this way can error-free contact lens edges be obtained.
The contact lenses produced in this manner are moulded parts having little mechanical stability and a water content of more than 60% by weight. After manufacture, the lens is inspected, then packed and subjected to heat sterilisation at 121° C. in an autoclave.
The materials used for these moulds are preferably plastics, e.g. polypropylene. The moulds are produced by injection moulding and are only used once. This is because, among other things, the moulds are partially contaminated by the surplus material, are damaged when the contact lens is separated or are irreversibly deformed in partial areas when the mould is closed. In particular, because of the quality requirements of the contact lenses edges, the moulds are only used once, since a certain amount of deformation of the moulds at the area of their edge cannot be excluded with certainty.
In U.S. Pat. No. 5,508,317, a new contact lens material is described, which represents an important improvement in the chemistry of polymerisable starting materials for the manufacture of contact lenses. The patent discloses a water-soluble composition of a prepolymer, which is filled into the mould cavity and then crosslinked photochemically. Since the prepolymer has several crosslinkable groups, the crosslinking is characterised by its high quality, so that a finished lens of optical quality can be produced within a few seconds, without the necessity for subsequent extraction or reworking steps. Owing to the improved chemistry of the starting material as illustrated in the patent, contact lenses can be produced at considerably lower cost, so that in this way it is possible to produce disposable lenses that are used only once.
EP-A-0 637 490 describes a process by means of which a further improvement may be obtained in the preparation process of contact lenses with the prepolymer described in U.S. Pat. No. 5,508,317, a derivative of a polyvinyl alcohol having a molecular weight of at least about 2,000 that, based on the number of hydroxy groups of the polyvinyl alcohol. Here, the material is filled into a mould comprising two halves, whereby the two mould halves do not touch, but a thin circular gap is located between them. The gap is linked to the mould cavity, so that surplus lens material can flow away into the gap. Crosslinking of the prepolymer takes place by radiation especially with UV light, whereby radiation is restricted to the mould cavity by a chromium mask. In this way, only the material in the mould cavity is crosslinked, so that there is high reproducibility of the edges of the lens without closing the two mould halves.
In this process, instead of the polypropylene moulds that may be used only once, reusable quartz/glass moulds are used. In addition, quartz is notable for its good UV permeability and is very hard and refractory.
However, moulds made from quartz or glass are very expensive to produce, so that owing to the high costs, the moulds ought to have quite long service life in order to ensure that the process is economical. Therefore, for economic reasons, only a limited number of variants can be realised during lens design and prototyping phase, for example in respect of the dioptre number.
In order to solve this problem, the intended objective is to replace at least one of the two mould halves, especially the female mould half, with a mould half consisting of plastic, and thus to employ one mould half made of plastic in combination with one mould half made of quartz or glass.
U.S. Pat. No. 7,845,615 has disclosed CR-39® (diallyl diglycol carbonate polymer) from PPG Industries could be employed as one mold half in combination with one mold half made of quartz or glass. Unfortunately, some issues such as dimensional stability over time, and mold cleaning were reported during the use of CR-39® as mold materials in the LightStream platform.
Therefore, there is still a need for new reusable molds for front curve mold that can be produced using single point diamond turning that is cost effective and short lead time while providing dimensional stability.